Sealable, flame-resistant, co-extruded, biaxially oriented foil that is mat on one side and a method for producing and using same

ABSTRACT

The invention relates to a sealable, flame-retardant, coextruded, biaxially oriented polyester film with one matt side, and composed of at least one base layer B and of, applied to the two sides of this base layer, a sealable outer layer A and a matt outer layer C. The film also comprises at least one flame retardant. The invention further relates to the use of the film and to a process for its production.

The invention relates to a sealable, flame-retardant, coextruded,biaxially oriented polyester film with one matt side, and composed of atleast one base layer B and of, applied to the two sides of this baselayer, a sealable outer layer A and a matt outer layer C. The film alsocomprises at least one flame retardant. The invention further relates tothe use of the film and to a process for its production.

BACKGROUND OF THE INVENTION

GB-A 1 465 973 describes a coextruded, two-layer polyester film in whichone layer is composed of isophthalic acid-containing and terephthalicacid-containing copolyesters and in which the other layer is composed ofpolyethylene terephthalate. The specification gives no usefulinformation concerning the sealing performance of the film. Lack ofpigmentation means that the process for producing the film is notreliable (the film cannot be wound) and that there are limitations onfurther processing of the film.

EP-A-0 035 835 describes a coextruded sealable polyester film which hasparticles admixed in the sealable layer to improve winding andprocessing performance, the median size of these particles exceeding thethickness of the sealable layer. The particulate additives form surfaceprotrusions which prevent undesired blocking and sticking to rollers orguides. No further detail is given concerning incorporation ofantiblocking agents in the other, non-sealable layer of the film. It isuncertain whether this layer comprises antiblocking agents. The sealingperformance of the film is impaired by selecting particles withdiameters greater than the sealing layer, and by selecting theconcentrations given in the examples. The specification gives noinformation on the sealing temperature range of the film. Seal seamstrength is measured at 140° C. and is in the range from 63 to 120 N/m(from 0.97 to 1.8 N/15 m of film width).

EP-A-0 432 886 describes a coextruded multilayer polyester film whichhas one surface on which a sealable layer has been arranged and a secondsurface on which an acrylate layer has been arranged. Here, too, thesealable outer layer may be composed of isophthalic acid-containing andterephthalic acid-containing copolyesters. The reverse-side coatinggives the film improved processing performance. The specification givesno information concerning the sealing range of the film. Seal seamstrength is measured at 140° C. For a sealing layer of thickness 11 μmthe seal seam strength given is 761.5 N/m (11.4 N/15 mm). A disadvantageof the reverse-side acrylate coating is that this side is not thensealable with respect to the sealable outer layer. The uses of the filmare therefore highly restricted.

EP-A-0 515 096 describes a coextruded, multilayer sealable polyesterfilm which comprises an additional additive on the sealable layer. Theadditive may comprise inorganic particles, for example, and ispreferably applied in an aqueous layer to the film during itsproduction. The result is said to be that the film retains good sealingproperties and processes well. The reverse side comprises only very fewparticles, which pass into this layer mainly via the regrind. Again, thespecification gives no information concerning the sealing temperaturerange of the film. Seal seam strength is measured at 140° C. and is morethan 200 N/m (3 N/15 mm). For a sealing layer of thickness 3 μm the sealseam strength given is 275 N/m (4.125 N/15 mm).

WO 98/06575 describes a coextruded multilayer polyester film whichcomprises a sealable outer layer and a non-sealable base layer. The baselayer here may be composed of one or more layers, the interior layerbeing in contact with the sealable layer. The other (exterior) layerthen forms the second non-sealable outer layer. Here, too, the sealableouter layer may be composed of isophthalic acid-containing andterephthalic acid-containing copolyesters, but no antiblocking particlesare present in these. In addition, the film also comprises at least oneUV absorber, which is added to the base layer in a weight ratio of from0.1 to 10%. The base layer has conventional antiblocking agents. Thefilm has good sealability, but does not have the desired processingperformance and also has shortcomings in optical properties. The filmmay also have a matt surface, but then has high haze, which isundesirable.

DE-B 23 46 787 describes a flame-retardant raw material. Besides the rawmaterial, its use to give films and fibers is also claimed. Thefollowing shortcomings were apparent during production of film usingthis claimed phospholane-modified raw material

the raw material mentioned is susceptible to hydrolysis and has to bevery effectively predried. When the raw material is dried using priorart dryers it cakes, and production of a film is possible only undervery difficult conditions,

the films produced, under uneconomic conditions, also embrittle at hightemperatures, i.e. the mechanical properties decline sharply as a resultof embrittlement, making the film unusable; this embrittlement arisesafter as little as 48 hours at high temperature.

It is an object of the present invention to eliminate the disadvantagesof the prior art.

BRIEF DESCRIPTION OF THE INVENTION

The invention provides a sealable, flame-retardant, coextruded,biaxially oriented polyester film with one matt side and with at leastone base layer B, and with a sealable outer layer A, and also with amatt outer layer C, where a flame retardant is present in at least onelayer, where the sealable outer layer A has a minimum sealingtemperature of 110° C. and a seal seam strength of at least 1.3 N/15 mm,and the topographies of the two outer layers A and C have the followingfeatures.

Sealable outer layer A:

R_(a)<30 nm

Value measured for gas flow from 500 to 4000 s

Non-sealable, matt outer layer C:

200 nm<R_(a)<1000 nm

Value measured for gas flow <50 s.

DETAILED DESCRIPTION OF THE INVENTION

The invention therefore provides a sealable, transparent,flame-retardant, sealable, coextruded and biaxially oriented polyesterfilm with one matt side and not having the disadvantages of the priorart films mentioned and having in particular very good sealability, andcapable of being produced cost-effectively, and which has improvedprocessability, and improved optical properties. In particular, it isflame-retardant and does not embrittle after exposure to hightemperature.

Success has been achieved in extending the sealing range of the film atlow temperatures, increasing the seal seam strength of the film, and atthe same time providing film handling which is better than that knownfrom the prior art. It has moreover been ensured that the film can alsobe processed on high-speed processing machinery. During production ofthe film it is possible to introduce directly-arising regrind at aconcentration of up to 60% by weight, based on the total weight of thefilm to the extrusion process without any significant resultant adverseeffect on the physical properties of the film.

Flame retardancy means that in what is known as a fire protection testthe transparent film meets the requirements of DIN 4102 Part 2 and inparticular the requirements of DIN 4102 Part 1 and can be assigned tobuilding materials classification B2 and in particular B1 forlow-flammability materials.

The film is also intended to pass the UL 94 test “Vertical Burning Testfor Flammability of Plastic Material”, thus permitting itsclassification in class 94 VTM-0. This means that the film has ceased toburn 10 seconds after removal of the Bunsen burner, and that after 30seconds no smoldering is observed, and moreover no burning drops areobserved.

Cost-effective production includes the capability of the raw materialsor the raw material components needed to produce the flame-retardantfilm to be dried using industrial dryers of the prior art. It issignificant that the raw materials do not cake and do not undergothermal degradation. These industrial dryers of the prior art includevacuum dryers, fluidized-bed dryers, and fixed-bed dryers (towerdryers).

These dryers operate at temperatures from 100 to 170° C., at which theflame-retardant raw materials of the prior art used hitherto generallycake and have to be dug out, making film production impossible.

In vacuum dryers, which have the gentlest drying action, the rawmaterial passes through a range of temperature of from about 30 to 130°C. at a reduced pressure of 50 mbar. A process known as post-drying isthen required, in a hopper at temperatures of from 100 to 130° C. andwith a residence time of from 3 to 6 hours. Even here, the known rawmaterial cakes to an extreme extent.

No embrittlement on short-term exposure to high temperature means thatafter 100 hours of heat-conditioning at 100° C. in a circulating-airdrying cabinet the film does not become brittle and does not have poormechanical properties.

Good mechanical properties include, inter alia, high modulus ofelasticity (E_(MD)>3200 N/mm²; E_(TD)>3500 N/mm²) and also values fortensile stress at break (in MD>100 N/mm²; in TD>130 N/mm²).

According to the invention, the film generally has at least threelayers, the layers then encompassed being the base layer B, the sealableouter layer A, and the matt outer layer C.

At least 90% by weight of the base layer B of the film is generallycomposed of a thermoplastic polyester. Polyesters suitable for thispurpose are those made from ethylene glycol and terephthalic acid(polyethylene terephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid(poly-1,4-cyclohexanedimethylene terephthalate, PCDT), or else made fromethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters atleast 90 mol %, preferably at least 95 mol %, of which is composed ofethylene glycol units and terephthalic acid units, or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. The remaining monomerunits derive from those other aliphatic, cycloaliphatic or aromaticdiols and, respectively, dicarboxylic acids which can also be present inlayer A (or layer C).

Other examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) and branchedaliphatic glycols having up to 6 carbon atoms. Among the cycloaliphaticdiols, mention should be made of cyclohexanediols (in particular1,4-cyclohexanediol). Examples of other suitable aromatic diols have theformula HO—C₆H₄—X—C₆H₄—OH, where X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—, —O—,—S— or —SO₂—. Bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also verysuitable.

Other aromatic dicarboxylic acids are preferably benzenedicarboxylicacids, naphthalene dicarboxylic acids (such as naphthalene-1,4- or-1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylicacids (in particular diphenylacetylene-4,4′-dicarboxylic acid) orstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids mention should be made of cyclohexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the C₃-C₁₉ alkanediacids are particularly suitable,and the alkane moiety here may be straight-chain or branched.

One way of preparing the polyesters is the transesterification process.Here, the starting materials are dicarboxylic esters and diols, whichare reacted using the customary transesterification catalysts, such asthe salts of zinc, of calcium, of lithium, of magnesium or of manganese.The intermediates are then polycondensed in the presence of well-knownpolycondensation catalysts, such as antimony trioxide or titanium salts.Another equally good preparation method is the direct esterificationprocess in the presence of polycondensation catalysts. This startsdirectly from the dicarboxylic acids and the diols.

The sealable outer layer A coextruded onto the base layer B has astructure based on polyester copolymers and is substantially composed ofamorphous copolyesters predominantly made of isophthalic acid units andof terephthalic acid units and of ethylene glycol units. The othermonomer units derive from those other aliphatic, cycloaliphatic, oraromatic diols and, respectively, dicarboxylic acids which can also bepresent in the base layer. Preferred copolyesters which provide thedesired sealing properties are those composed of ethylene terephthalateunits and of ethylene isophthalate units, and of ethylene glycol units.The proportion of ethylene terephthalate is from 40 to 95 mol % and thecorresponding portion of ethylene isophthalate is from 60 to 5 mol %.Preference is given to copolyesters where the proportion of ethyleneterephthalate is from 50 to 90 mol % and the corresponding proportion ofethylene isophthalate is from 50 to 10 mol %, and highly preferredcopolyesters are those where the proportion of ethylene terephthalate isfrom 60 to 85 mol % and the corresponding proportion of ethyleneisophthalate is from 40 to 15 mol %.

In its preferred embodiment, the matt outer layer C comprises a blend ora mixture made from two components I and II and, where appropriate,comprises added additives in the form of inert inorganic antiblockingagents.

Component I of the mixture or of the blend is a polyethyleneterephthalate homopolymer or polyethylene terephthalate copolymer, or amixture made from polyethylene terephthalate homo- or copolymers.

Component II of the copolymer or of the mixture or of the blend is apolyethylene terephthalate copolymer which is composed of thecondensation product of the following monomers or of their derivativescapable of forming polyesters:

A) from 65 to 95 mol % of isophthalic acid;

B) from 0 to 30 mol % of at least one aliphatic dicarboxylic acid havingthe formula HOOC(CH₂)_(n)COOH, where n is in the range from 1 to 11;

C) from 5 to 15 mol % of at least one sulfomonomer containing an alkalimetal sulfonate group on the aromatic moiety of a dicarboxylic acid;

D) a copolymerizable aliphatic or cycloaliphatic glycol having from 2 to11 carbon atoms, in the stoichiometric amount necessary to form 100 mol% of condensate;

where each of the percentages is based on the total amount of monomersforming component II. For a detailed description of component II seealso EP-A-0 144 878, which is expressly incorporated herein by way ofreference.

For the purposes of the present invention, mixtures are mechanicalmixtures prepared from the individual components. For this, theindividual constituents are generally combined in the form ofsmall-dimensioned compressed moldings, e.g. lenticular or bead-shapedpellets, and mixed with one another mechanically, using a suitableagitator. Another way of producing the mixture is to feed components Iand II in pellet form separately to the extruder for the outer layer ofthe invention, and to carry out mixing in the extruder and/or in thedownstream systems for transporting the melt.

For the purposes of the present invention, a blend is an alloy-likecomposite of the individual components I and II which can no longer beseparated into the initial constituents. A blend has properties likethose of a homogeneous material and can therefore be characterized byappropriate parameters.

The ratio (ratio by weight) of the two components I and II of the outerlayer mixture or of the blend can be varied within wide limits anddepends on the intended use of the multilayer film. The ratio ofcomponents I and II is preferably in the range from I:II=10:90 toI:II=95:5, preferably from I:II=20:80 to I:II=95:5, and in particularfrom I:II=30:70 to I:II=95:5.

The desired sealing properties, the desired degree of mattness, and thedesired processing properties of the film of the invention are obtainedby combining the properties of the copolyester used for the sealableouter layer with the topographies of the sealable outer layer A and ofthe non-sealable, matt outer layer C.

The minimum sealing temperature of 110° C. and the seal seam strength ofat least 1.3 N/15 mm is achieved if the copolymers described in moredetail above are used for the sealable outer layer A. The best sealingproperties are obtained for the film if no other additives, inparticular no inorganic or organic fillers, are added to the copolymer.The lowest minimum sealing temperature and the highest seal seamstrengths are then obtained for a prescribed copolyester. However, thefilm then has poor handling, since the surface of the sealable outerlayer A has a severe tendency toward blocking. The film is difficult towind and is unsuitable for further processing on high-speed packagingmachinery. To improve the handling of the film and its processability itis necessary to modify the sealable outer layer A. This is best achievedwith the aid of suitable antiblocking agents of a selected size, acertain concentration of which is added to the sealable layer, andspecifically in such a way as firstly to minimize blocking and secondlyto give only insignificant impairment of sealing properties. Thisdesired combination of properties may be achieved if the topography ofthe sealable outer layer A is characterized by the following parameterset:

The roughness of the sealable outer layer, characterized by the R_(a)value, is generally smaller than 30 nm, preferably smaller than 25 nm.Otherwise, the sealing properties for the purposes of the presentinvention are adversely affected.

The value measured for the gas flow is to be in the range from 500 to4000 s, preferably from 600 to 3500 s. At values below 500 s, thesealing properties are adversely affected for the purposes of thepresent invention, and at values above 4000 s the handling of the filmbecome poor.

The non-sealable, matt outer layer C is characterized by the followingparameter set

The roughness of the matt outer layer, characterized by the R_(a) value,is in the range from 200 to 1000 nm, preferably from 220 to 900 nm.Smaller values than 200 nm have adverse effects on the windingperformance and the processing performance of the film, and also on thedegree of mattness of the surface. Greater values than 1000 nm impairthe optical properties (haze) of the film.

The value measured for gas flow should be in the range ≦50 s, preferably≦45 s. At values above 50, the degree of mattness of the film isadversely affected.

The film of the invention comprises at least one flame retardant, whichis metered in by way of what is known as masterbatch technology directlyduring film production, the concentration of the flame retardant beingin the range from 0.5 to 30.0% by weight, preferably from 1.0 to 20.0%by weight, based on the weight of the layer of the crystallizablethermoplastic. The ratio of flame retardant to thermoplastic duringproduction of the masterbatch is generally kept within from 60:40% byweight to 10:90% by weight.

Typical flame retardants include bromine compounds, chloroparaffins andother chlorine compounds, antimony trioxide, and aluminum trihydrates,but the halogen compounds are disadvantageous due to the production ofhalogen-containing by-products. Other serious disadvantages are the lowlightfastness of a film in which they are present, and the evolution ofhydrogen halides in the event of a fire.

Examples of suitable flame retardants used according to the inventionare organophosphorus compounds, such as carboxyphosphinic acids,anhydrides of these, and dimethyl methylphosphonate. It is significantfor the invention that the organophosphorus compound is soluble in thethermoplastic, since otherwise the requirements for optical propertiesare not met.

Since the flame retardants generally have some susceptibility tohydrolysis, it can be advisable to make concomitant use of a hydrolysisstabilizer.

It was therefore more than surprising that the use of masterbatchtechnology and of appropriate predrying and/or precrystallization, and,where appropriate, use of small amounts of a hydrolysis stabilizerpermit economic production of a flame-retardant film with the requiredproperty profile without caking in the dryer, and that the film does notembrittle after exposure to high temperature. In addition, neither anyevolution of gases nor any formation of deposits was observed in theproduction process.

It was very surprising that alongside this excellent result with therequired flame retardancy

There is no adverse effect on the Yellowness Index of the film, withinthe limits of accuracy of measurement, when comparison is made with anunmodified film.

Neither any evolution of gases nor any formation of die deposits nor anycondensation of vapor onto frames occurs, and therefore the film hasexcellent optical properties, excellent profile, and extremely goodlayflat.

The flame-retardant film has extremely good stretchability, and istherefore capable of stable production in a reliable process onhigh-speed film lines at speeds of up to 420 m/min.

A film of this type is therefore also cost-effective.

It is moreover very surprising that it is even possible to reuse theregrind generated from the films or from the moldings without anyadverse effect on the Yellowness Index of the film.

In one preferred embodiment, the film of the invention comprises acrystallizable polyethylene terephthalate as main constituent, and asflame retardant from 1.0 to 20.0% by weight of an organophosphoruscompound soluble in the polyethylene terephthalate, and from 0.1 to 1.0%by weight of a hydrolysis stabilizer.

Phenolic stabilizers, alkali metal/alkaline earth metal stearates,and/or alkali metal/alkaline earth metal carbonates are particularlysuitable. Preference is given to phenolic stabilizers in amounts of from0.05 to 0.6% by weight, in particular from 0.15 to 0.3% by weight, andwith a molar mass of more than 500 g/mol. Pentaerythrityltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate or1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene areparticularly advantageous.

In the three-layer embodiment, the flame retardant is preferably presentin the non-sealable outer layer C. However, if required, there may alsobe flame retardants in the base layer B or in the sealable outer layerA. The concentration of the flame retardant(s) is based here on theweight of the thermoplastics in the layer provided with flameretardants.

Very surprisingly, fire protection tests to DIN 4102 and the UL testhave shown that in the case of a three-layer film it is fully sufficientto provide flame retardant in the outer layers of thickness from 0.3 to2.5 μm in order to achieve improved flame retardancy. If required, andif fire-protection requirements are stringent, the core layer may alsohave flame retardant, at what is known as a base level of provision.

The flame-retardant, multilayer films produced by known coextrusiontechnology are therefore of economic interest when compared withmonofilms bulk-modified at high concentrations, since markedly lessflame retardant is needed.

Furthermore, measurements have shown that the film of the invention doesnot embrittle on exposure to temperatures of 100° C. for a prolongedperiod. This is more than surprising. This result is attributable to thesynergistic action of appropriate precrystallization, predrying,masterbatch technology, and hydrolysis stabilizer.

Furthermore, the film is capable of problem-free recycling withoutpollution of the environment and without loss of mechanical properties,making it suitable for use as short-lived advertising placards, forexample, or in the construction of exhibition stands, or for otherpromotional items, where fire protection is desired.

Surprisingly, even films of the invention in the thickness range from 5to 300 μm give compliance with construction material specifications B2and B1 to DIN 4102 and with the UL 94 test.

According to the invention, the flame retardant is added by way ofmasterbatch technology. The flame retardant, and the hydrolysisstabilizer where appropriate, are first completely dispersed in acarrier material. Carrier materials which may be used are thethermoplastic itself, e.g. the polyethylene terephthalate, or else otherpolymers which are compatible with the thermoplastic. After meteringinto the thermoplastic for film production the constituents of themasterbatch melt during the extrusion process and are thus dispersed inthe thermoplastic.

In masterbatch technology it is important that the grain size and thebulk density of the masterbatch is similar to the grain size and thebulk density of the thermoplastic, permitting homogeneous distributionand therefore homogeneous flame retardancy.

It is significant for the invention that the masterbatch which comprisesthe flame retardant is precrystallized or predried. This predryingincludes gradual heating of the masterbatch at subatmospheric pressure(from 20 to 80 mbar, preferably from 30 to 60 mbar, in particular from40 to 50 mbar), with stirring, and, where appropriate, after-drying at aconstant elevated temperature, likewise at subatmospheric pressure. Itis preferable for the masterbatch to be charged at room temperature froma metering vessel in the desired blend together with the polymer of thebase and/or outer layers and, where appropriate, with other raw materialcomponents batchwise into a vacuum dryer which traverses a temperatureprofile from 10 to 160° C., preferably from 20 to 150° C., in particularfrom 30 to 130° C., during the course of the drying time or residencetime. During the residence time of about 6 hours, preferably 5 hours, inparticular 4 hours, the raw material mixture is stirred at from 10 to 70rpm, preferably from 15 to 65 rpm, in particular from 20 to 60 rpm. Theresultant precrystallized or predried raw material mixture isafter-dried in a downstream vessel, likewise evacuated, at temperaturesof from 90 to 180° C., preferably from 100 to 170° C., in particularfrom 110 to 160° C., for from 2 to 8 hours, preferably from 3 to 7hours, in particular from 4 to 6 hours.

The base layer B may also comprise conventional additives, such asstabilizers and/or antiblocking agents. The two other layers A and C mayalso comprise these additives. They are advantageously added to thepolymer or to the polymer mixture before melting begins. Examples ofstabilizers used are phosphorus compounds, such as phosphoric acid orphosphoric esters.

Typical antiblocking agents (in this context also termed pigments) areinorganic and/or organic particles, such as calcium carbonate, amorphoussilica, talc, magnesium carbonate, barium carbonate, calcium sulfate,barium sulfate, lithium phosphate, calcium phosphate, magnesiumphosphate, aluminum oxide, LiF, the calcium, barium, zinc or manganesesalts of the dicarboxylic acids used, carbon black, titanium dioxide,kaolin, crosslinked polystyrene particles, and crosslinked acrylateparticles.

Other antiblocking agents which may be selected are mixtures of two ormore different antiblocking agents, and mixtures of antiblocking agentsof identical makeup but different particle size. The particles may beadded to each layer of the film in the respective advantageousconcentrations, e.g. as a glycolic dispersion during polycondensation,or by way of masterbatches during extrusion.

Preferred particles are SiO₂ in colloidal or chain-type form. Theseparticles are very effectively bonded into the polymer matrix andgenerate only very few vacuoles. Vacuoles generally cause haze and aretherefore advantageously avoided. There is no restriction in principleon the diameters of the particles used. However, for achieving theobject it has proven advantageous to use particles with an averageprimary particle diameter smaller than 100 nm, preferably smaller than60 nm, and particularly preferably smaller than 50 nm, and/or particleswith an average primary particle diameter greater than 1 μm, preferablygreater than 1.5 μm, and particularly preferably greater than 2 μm.These latter particles should not, however, have an average particlediameter greater than 5 μm.

To achieve the abovementioned properties of the sealable film, it hasproven advantageous if the particle concentration selected in the baselayer B is lower than in the two outer layers A and C. In the case of athree-layer film of the type mentioned, the particle concentration inthe base layer B will be in the range from 0 to 0.15% by weight,preferably from 0.001 to 0.12% by weight, and in particular from 0.002to 0.10% by weight. There is no restriction in principle on the diameterof the particles used, but particular preference is given to particleswith an average diameter greater than 1 mm.

In the advantageous embodiment, the film is composed of three layers,the base layer B and, applied to the two sides of this base layer, outerlayers A and C, the outer layer A being sealable with respect to itselfand with respect to the outer layer C.

To achieve the property profile mentioned for the film, the outer layerC has more pigment (i.e. higher pigment concentration) than the outerlayer A. The pigment concentration in this second, matt outer layer C isin the range from 1.0 to 10.0%, advantageously from 1.5 to 10%, and inparticular from 2.0 to 10%. In contrast, the other sealable outer layerA, opposite to the outer layer C, has a lower filler level of inertpigments. The concentration of the inert particles in the layer A is inthe range from 0.01 to 0.2% by weight, preferably from 0.015 to 0.15% byweight, and in particular from 0.02 to 0.1% by weight.

Where appropriate, there may also be an intermediate layer between thebase layer and the outer layers. This may again be composed of thepolymers described for the base layers. In one particularly preferredembodiment, the intermediate layer is composed of the polyester used forthe base layer. It may also comprise the conventional additivesdescribed. The thickness of the intermediate layer is generally greaterthan 0.3 μm and is preferably in the range from 0.5 to 15.0 μm,particularly in the range from 1.0 to 10.0 μm, and very preferably inthe range from 1.0 to 5.0 μm.

In the particularly advantageous three-layer embodiment of the film ofthe invention, the thickness of the outer layers A and C is generallygreater than 0.1 μm and is generally in the range from 0.2 to 4.0 μm,advantageously from 0.2 to 3.5 μm, in particular from 0.3 to 3 μm, andvery particularly preferably from 0.3 to 2.5 μm, and the thicknesses ofthe outer layers A and C here may be identical or different.

The total thickness of the film of the invention may vary within certainlimits. It is from 3 to 100 μm, in particular from 4 to 80 μm,preferably from 5 to 70 μm, the proportion made up by the layer Bpreferably being from 5 to 90% of the total thickness.

The polymers for the base layer B and the two outer layers A and C arefed to three extruders. Any foreign bodies or contamination present maybe removed from the polymer melt by filtration prior to extrusion. Themelts are then extruded through a coextrusion die to give flat meltfilms and layered one upon the other. The multilayer film is then drawnoff and solidified with the aid of a chill roll and, where appropriate,other rollers.

The film of the invention is generally used by the coextrusion processknown per se.

The procedure for this process is that the melts corresponding to theindividual layers of the film are coextruded through a flat-film die,the resultant film is drawn off for solidification on one or morerollers, the film is then biaxially stretched (oriented), and thebiaxially stretched film is heated and set and, where appropriate,corona- or flame-treated on the surface layer intended for treatment.

The biaxial stretching (orientation) is generally carried outsequentially, and preference is given to sequential biaxial stretchingin which stretching is first longitudinal (in the machine direction) andthen transverse (perpendicular to the machine direction).

As is usual in coextrusion, the polymer or the polymer mixture for theindividual layers is first compressed and plasticized in an extruder,and any additives used may already be present in the polymer or thepolymer mixture. The melts are then simultaneously extruded through aflat-film die (slot die), and the coextruded film is drawn off on one ormore take-off rolls, whereupon the film cools and solidifies.

The biaxial orientation is generally carried out sequentially,preferably orienting first longitudinally (i.e. in the machinedirection=MD) and then transversely (i.e. perpendicularly to the machinedirection=TD). This gives orientation of the molecular chains. Thelongitudinal orientation can be carried out with the aid of two rollsrunning at different speeds corresponding to the desired stretchingratio. For the transverse orientation use is generally made of anappropriate tenter frame.

The temperature at which the orientation is carried out may vary over arelatively wide range and depends on the film properties desired. Thelongitudinal stretching is generally carried out at from about 80 to130° C., and the transverse stretching at from about 90 to 150° C. Thelongitudinal stretching ratio is generally in the range from 2.5:1 to6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio isgenerally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to4.5:1. Prior to the transverse stretching, one or both surfaces of thefilm may be in-line coated by known processes. The in-line coating mayserve, for example, to give improved adhesion of the metal layer or ofany printing ink subsequently to be applied, or else to improveantistatic performance or processing performance.

For producing a film with very good sealing properties, it has provenfavorable for the planar orientation Δp of the film to be less than0.165, in particular less than 0.163. In this instance, the strength ofthe film in the direction of its thickness is sufficiently great thatwhen the seal seam strength is measured it is specifically the seal seamwhich is pulled apart, and there is no initiation or propagation oftearing within the film.

The significant variables affecting the planar orientation Δp have beenfound to be the longitudinal and transverse stretching parameters, andalso the SV of the raw material used. The processing parameters includein particular the longitudinal and transverse stretching ratios (λ_(MD)and λ_(TD)), the longitudinal and transverse stretching temperatures(T_(MD) and T_(TD)), the film web speed and the nature of thestretching, in particular that in the longitudinal direction of themachine. For example, if a machine gives a planar orientation Δp=0.167with the following set of parameters: λ_(MD)=4.8 and λ_(TD)=4.0, andlongitudinal and transverse stretching temperatures T_(MD)=from 80 to118° C. and T_(TD)=from 80 to 125° C., then increasing the longitudinalstretching temperature to T_(MD)=from 80 to 125° C., or increasing thetransverse stretching temperature to T_(TD)=from 80 to 135° C., orlowering the longitudinal stretching ratio to λ_(MD)=4.3, or loweringthe transverse stretching ratio to λ_(TD)3.7 gives a planar orientationΔp within the desired range. The film web speed here was 340 m/min, andthe SV of the material was about 730. In the case of longitudinalstretching, the data specified are based on what is known as N-TEPstretching, which is composed of a low-orientation stretching step(LOE=low-orientation elongation) and of a high-orientation stretchingstep (REP=rapid elongation process). With other stretching systems, theconditions are in principle the same, but the values for the respectiveprocess parameters may be slightly different. In the case oflongitudinal stretching, the temperatures given are based on therespective roll temperatures, and in the case of transverse stretchingthey are based on the film temperatures measured by IR.

In the heat-setting which follows, the film is held for from about 0.1to 10 s at a temperature of from 150 to 250° C. The film is then woundup in a conventional manner

One or both surfaces of the film is/are preferably corona- orflame-treated by one of the known methods after biaxial stretching. Theintensity of treatment is generally in the range above 45 mN/m.

The film may also be coated to establish other desired properties.Typical coatings are layers with adhesion-promoting, antistatic,slip-enhancing or release effect. It is, of course, possible for theseadditional layers to be applied to the film by in-line coating usingaqueous dispersions prior to the transverse stretching step.

The film of the invention has excellent sealability, very good flameretardancy, very good handling, and very good processing performance.The sealable outer layer A of the film seals not only with respect toitself (fin sealing) but also with respect to the non-sealable outerlayer C (lap sealing). The upward shift in minimum sealing temperatureof lap sealing is only about 10 K, and the seal seam strength isinferior by not more than 0.3 N/15 mm.

In addition, it was possible to increase mattness while at the same timereducing the haze of the film with respect to films of the prior art. Ithas been ensured that during production of the film regrind can bereintroduced to the extrusion process at a concentration of from 20 to60% by weight, based on the total weight of the film, without anysignificant resultant adverse effect on the physical properties of thefilm.

Due to the excellent sealing properties of the film, and due to its verygood handling and very good processing properties, the film isparticularly suitable for processing on high-speed machinery.

Due to its excellent combinations of properties, the film is moreoversuitable for many varied applications, such as for interior decoration,for the construction of exhibition stands, for exhibition requisites, asdisplays, for placards, for protective glazing of machinery or ofvehicles, in the lighting sector, in the fitting out of shops or ofstores, as a promotional item or a laminating medium, or outdoors forgreenhouses, roofing systems, exterior cladding, protective coveringsfor materials, such as steel sheets, construction sector applications,or for illuminated advertising profiles, blinds, or electricalapplications.

The films and items produced therefrom are particularly suitable foroutdoor applications where there is a requirement for fire protection orflame retardancy.

The outer layer C has a characteristic matt, non-reflective surface, andis therefore particularly attractive for the applications mentioned.

The table below (Table 1) gives the most important properties of thefilm of the invention.

TABLE 1 Range of the particularly invention preferred preferred UnitTest method Outer layer A Minimum sealing <110 <105 <100 ° C. internaltemperature Seal seam strength >1.3 >1.5 >1.8 N/15 mm internal Averageroughness R_(a) <30 <25 <20 nm DIN 4768, cut- off 0.25 mm Range ofvalues measured 500-4000 800-3500 1000-3000 sec internal for gas flowGloss, 20° >120 >130 >140 DIN 67 530 Outer layer C COF <0.5 <0.45 <0.40DIN 53 375 Average roughness R_(a) 200-1000 225-900  250-800 nm DIN4768, cut- off 0.25 mm Range of values measured <50 <45 <49 sec internalfor gas flow Gloss, 60° <60 <55 <50 DIN 67 530 Other film propertiesHaze <40 <35 <30 % ASTM-D 1003-52 Planar orientation <0.1650 <0.163<0.160 internal Fire Performance The film complies with constructionmaterials classifications B2 and B1 to DIN 4102 Part 2/Part 1 and passesthe UL 94 test

In the examples below, each property was measured in accordance with thefollowing standards or methods.

SV (DCA), IV (DVA)

Standard viscosity SV (DCA) is measured in dichloroacetic acid by amethod based on DIN 53726.

Intrinsic viscosity (IV) is calculated as follows from standardviscosity

IV(DCA)=6.67·10⁻⁴ SV(DCA)+0.118

Determination of Minimum Sealing Temperature

Hot-sealed specimens (seal seam 20 mm×100 mm) were produced using aBrugger HSG/ET sealing apparatus, by sealing the film at differenttemperatures with the aid of two heated sealing jaws at a sealingpressure of 2 bar and with a sealing time of 0.5 s. From the sealedspecimens, test strips of 15 mm width were cut. The T-seal seam strengthwas measured in the determination of seal seam strength. The minimumsealing temperature is the temperature at which a seal seam strength ofat least 0.5 N/15 mm is achieved.

Seal Seam Strength

To determine seal seam strength, two film strips of width 15 mm wereplaced one on top of the other and sealed at 130° C. with a sealing timeof 0.5 s and a sealing pressure of 2 bar (apparatus: Brugger model NDS,single-side-heated sealing jaw). The seal seam strength was determinedby the T-Peel method.

Coefficient of Friction (COF)

Coefficient of friction was determined to DIN 53 375. The coefficient ofsliding friction was measured 14 days after production.

Surface Tension

Surface tension was determined by what is known as the ink method (DIN53 364).

Haze

Hölz haze was measured by a method based on ASTM-D 1003-52 but, in orderto utilize the most effective measurement range, measurements were madeon four pieces of film laid one on top of the other, and a 1° slitdiaphragm was used instead of a 4° pinhole.

Gloss

Gloss was determined to DIN 67 530. Reflectance was measured, as anoptical value characteristic of a film surface. Based on the standardsASTM-D 523-78 and ISO 2813, the angle of incidence was set at 20°. Abeam of light hits the flat test surface at the set angle of incidenceand is reflected and/or scattered thereby. A proportional electricalvariable is displayed representing light rays hitting thephotoelectronic detector. The value measured is dimensionless and mustbe stated together with the angle of incidence.

Surface Gas Flow Time

The principle of the test method is based on the air flow between oneside of the film and a smooth silicon wafer sheet. The air flows fromthe surroundings into an evacuated space, and the interface between filmand silicon wafer sheet acts as a flow resistance.

A round specimen of film is placed on a silicon wafer sheet in themiddle of which there is a hole providing the connection to thereceiver. The receiver is evacuated to a pressure below 0.1 mbar. Thetime in seconds taken by the air to establish a pressure rise of 56 mbarin the receiver is determined.

Test conditions: Test area 45.1 cm¹ Weight applied 1276 g Airtemperature 23° C. Humidity 50% relative humidity Aggreagated gas volume1.2 cm² Pressure difference 56 mbar

Determination of Planar Orientation Δp

Planar orientation is determined by measuring the refractive index withan Abbe refractometer.

Preparation of specimens Specimen size and length: from 60 to 100 mmSpecimen width: corresponds to prism width of 10 mm

To determine n_(MD) and n_(a) (=n_(z)), the specimen to be tested has tobe cut out from the film with the running edge of the specimen runningprecisely in the direction TD. To determine n_(TD) and n_(a) (=n₂), thespecimen to be tested has to be cut out from the film, and the runningedge of the specimen has to coincide exactly with the direction MD. Thespecimens are to be taken from the middle of the film web. Care must betaken that the temperature of the Abbe refractometer is 23° C. Using aglass rod, a little diiodomethane (N=1.745) ordiiodomethane-bromonaphthalene mixture is applied to the lower prism,which has been cleaned thoroughly before the test. The refractive indexof the mixture must be greater than 1.685. The specimen cut out in thedirection TD is firstly laid on top of this, in such a way that theentire surface of the prism is covered. Using a paper wipe the film isnow firmly pressed flat onto the prism, so that it is firmly andsmoothly positioned thereon. The excess liquid must be sucked away. Alittle of the test liquid is then dropped onto the film. The secondprism is swung down and into place and pressed firmly into contact. Theright-hand knurled screw is then used to turn the indicator scale untila transition from light to dark can be seen in the field of view in therange from 1.62 to 1.68. If the transition from light to dark is notsharp, the colors are brought together using the upper knurled screw insuch a way that only one light and one dark zone are visible. The sharptransition line is brought to the crossing point of the two diagonallines (in the eyepiece) using the lower knurled screw. The value nowindicated on the measurement scale is read off and entered into the testrecord. This is the refractive index n_(MD) in the machine direction.The scale is now turned using the lower knurled screw until the rangevisible in the eyepiece is from 1.49 to 1.50.

The refractive index n_(a) or n_(z) (in the direction of the thicknessof the film) is then determined. To improve the visibility of thetransition, which is only weakly visible, a polarization film is placedover the eyepiece. This is turned until the transition is clearlyvisible. The same considerations apply as in the determination ofn_(MD). If the transition from light to dark is not sharp (colored), thecolors are brought together using the upper knurled screw in such a waythat a sharp transition can be seen. This sharp transition line isbrought into the crossing point of the two diagonal lines using thelower knurled screw, and the value indicated on the scale is read offand entered into the table.

The specimen is then turned, and the corresponding refractive indicesn_(MD) and n_(a) (=n_(z)) of the other side are measured and enteredinto an appropriate table.

After determining the refractive indices in, respectively, the directionMD and the direction of the thickness of the film, the specimen stripcut out in the direction MD is placed in position and the refractiveindices n_(TD) and n_(a) (=n_(z)) are determined accordingly. The stripis turned over, and the values for the B side are measured. The valuesfor the A side and the B side are combined to give average refractiveindices. The orientation values are then calculated from the refractiveindices using the following formulae:

Δn=n _(MD) −n _(TD)

Δp=(n _(MD) +n _(TD))/2−n _(z)

n _(av)=(n _(MD) +n _(TD) +n _(z))/3

Surface Defects

Surface defects were determined visually.

Mechanical Properties

Modulus of elasticity, tensile stress at break, and tensile strain atbreak are measured longitudinally and transversely to ISO 527-1-2.

Yellowness Index

Yellowness Index (YI) is the deviation from colorlessness in the“yellow” direction and was measured to DIN 6167. Yellowness Indexvalues<5 are not visually detectable.

Fire Performance

Fire performance is determined to DIN 4102 Part 2, constructionmaterials classification B2, and to DIN 4102 Part 1, constructionmaterials classification B1, and also by the UL 94 test.

EXAMPLES

The examples and comparative examples below each concern films ofvarying thickness, produced by a known extrusion process.

Example 1

Chips made from polyethylene terephthalate (produced by thetransesterification process using Mn as transesterification catalyst, Mnconcentration: 100 ppm) were dried at 150° C. to residual moisture below100 ppm, and fed to the extruder for the base layer B. Similarly, chipsmade from polyethylene terephthalate and a filler were fed to theextruder for the non-sealable outer layer C.

Alongside, chips were produced from a linear polyester composed of anamorphous copolyester comprising 78 mol % of ethylene terephthalate and22 mol % of ethylene isophthalate (prepared by the transesterificationprocess using Mn as transesterification catalyst, Mn concentration: 100ppm). The copolyester was dried at a temperature of 100° C. to residualmoisture below 200 ppm and fed to the extruder for the sealable outerlayer A.

The hydrolysis stabilizer and the flame retardant are fed in the form ofa masterbatch. The masterbatch is composed of 20% by weight of flameretardant, 1% by weight of hydrolysis stabilizer, and 79% by weight ofpolyethylene terephthalate. The hydrolysis stabilizer is pentaerythritoltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. The flameretardant is dimethyl phosphonate ®Armgard P 1045). The masterbatch hasbulk density of 750 kg/m³ and a softening point of 69° C.

The masterbatch was charged at room temperature from separate feedvessels within a vacuum dryer which from the time of charging to the endof the residence time traverses a temperature profile from 25 to 130° C.During the residence time of about 4 hours, the masterbatch is stirredat 61 rpm. The precrystallized or predried masterbatch is after-dried inthe downstream hopper, likewise at subatmospheric pressure, for 4 hoursat 140° C.

10% by weight of the masterbatch are added to the base layer B, and 20%by weight of the masterbatch are added to the non-sealable outer layerC.

Coextrusion followed by stepwise longitudinal and transverse orientationwas used to produce a transparent three-layer film with ABC structureand a total thickness of 12 μm. Table 2 gives the thickness of each ofthe outer layers.

Outer layer A, a mixture made from: 97.0% by weight of copolyester withSV 800  3.0% by weight of masterbatch made from 97.75% by weight ofcopolyester (SV 800) smf 1.0% by weight of ® Sylobloc 44 H (syntheticSiO₂ from Grace), and 1.25% by weight of ® Aerosil TT 600 (fumed SiO₂from Degussa) Base layer B: 90.0% by weight of polyethyleneterephthalate with SV 800 10.0% by weight of masterbatch which comprisesflame retardant and hydrolysis stabilizer Outer layer C, a mixture madefrom: 20.0% by weight of masterbatch which comprises flame retardant andhydrolysis stabilizer, 65.0% by weight of polyethylene terephthalatewith SV 800 (= component I) 15.0% by weight of component II

Component II was prepared as described in Example 1 of EP-A-0 144 878.

The production conditions in each of the steps of the process were:

Extrusion: Temperatures Layer A: 270° C. Layer B: 290° C. Layer C: 290°C. Die gap width: 2.5 mm Take-off roll temperature: 30° C. LongitudinalTemperature: 80-125° C. stretching: Longitudinal stretching ratio: 4.2Transverse Temperature: 80-135° C. stretching: Transverse stretchingratio: 4.0 Setting: Temperature: 230° C. Duration: 3 s

The film had the required good sealing properties, and the desiredmattness, and has the desired handling and the desired processingperformance. Tables 2 and 3 show the structure of the films and theproperties achieved in films produced in this way.

The mechanical properties are unaltered after 200 hours ofheat-conditioning at 100° C. in a circulating-air drying cabinet. Thefilm exhibits no embrittlement phenomena of any type. The film complieswith construction materials classifications B2 and B1 to DIN 4102 Part 2and Part 1. The film passes the UL 94 test.

Example 2

Using Example 1 as a basis, the outer layer thickness for the sealablelayer A was raised from 1.5 to 2.0 μm. The result is an improvement insealing properties, and in particular a marked increase in seal seamstrength.

Example 3

Using Example 1 as a basis, the film now produced has a thickness of 20μm. The outer layer thickness for the sealable layer A was 2.5 μm andthat for the non-sealable layer C was 2.0 μm. The result was again animprovement in sealing properties, in particular a marked increase inseal seam strength. There was a marginal improvement in the handling ofthe film.

Example 4

Using Example 3 as a basis, the copolymer for the sealable outer layer Awas changed. Instead of the amorphous copolyester having 78 mol % ofpolyethylene terephthalate and 22 mol % of ethylene terephthalate, usewas now made of an amorphous copolyester having 70 mol % of polyethyleneterephthalate and 30 mol % of ethylene terephthalate. The raw materialwas processed in a vented twin-screw extruder, with no need forpredrying. The outer layer thickness of the sealable layer A was again2.5 μm and that of the non-sealable layer C was 2.0 μm. The result wasan improvement in sealing properties, in particular a marked increase inseal seam strength. To achieve good handling and good processingperformance of the film, the pigment concentration in the two outerlayers was slightly raised.

Comparative Example 1

Using Example 1 as a basis, the sealable outer layer A was now notpigmented. Although the result was some improvement in sealingproperties, there was an unacceptable deterioration in the handling ofthe film and the processing performance.

Comparative Example 2

Using Example 1 as a basis, the sealable outer layer A was now pigmentedat the same level as the non-sealable outer layer C. This measureimproved the handling and the processing properties of the film, butthere was a marked deterioration in the sealing properties.

Comparative Example 3

Using Example 1 as a basis, a markedly lower pigment level was now usedin the non-sealable outer layer A. There was a marked deterioration inthe handling of the film and the processing performance of the film.

Comparative Example 4

Example 1 of EP-A-0 035 835 was repeated. The sealing performance of thefilm, the handling of the film, and the processing performance of thefilm is poorer than in the inventive examples.

TABLE 2 Average pigment Layer diameter in Film thicknesses layersPigment concentrations thickness Film μm Pigments in layers μm ppmExample μm structure A B C A B C A B C A B C E1 12 ABC 1.5 9 1.5Sylobloc 44 H none 0 2.5 2.5 3e + 05 0 0 Aerosil TT 0.04 0.04 600 E2 12ABC 2 8.5 1.5 Sylobloc 44 H none 0 2.5 2.5 3e + 05 0 0 Aerosil TT 0.040.04 600 E3 20 ABC 2.5 16 2 Sylobloc 44 H none 0 2.5 2.5 3e + 05 0 0Aerosil TT 0.04 0.04 600 E4 20 ABC 2.5 16 2 Sylobloc 44 H none 0 2.5 2.54e + 05 0 0 Aerosil TT 0.04 0.04 600 CE1 12 ABC 1.5 9 1.5 none noneSylobloc 44 H 2.5 0 12001500 Aerosil TT 0.04 600 CE2 12 ABC 1.5 9 1.5Sylobloc 44 H none Sylobloc 44 H 2.5 2.5 3e + 05 0 12001500 Aerosil TTAerosil TT 0.04 0.04 600 600 CE3 12 ABC 1.5 9 1.5 Sylobloc 44 H noneSylobloc 44 H 2.5 2.5 3e + 05 0  600750 Aerosil TT Aerosil TT 0.04 0.04600 600 CE4 15 AB 2.3 13 Gasil 35 none 3 2500 0 EP-A 035 835

TABLE 3 Minimum Coefficient sealing of tempera- Seal seam friction turestrength COF Average Values Side A Side A Side C roughness measuredWinding Pro- with with with R_(a) for gas flow Gloss perform- cessingrespect respect respect Side Side Side Side Side Side ance and perform-Example to side A to side A to side C A C A C Δp A C Haze handling anceE1 100 2 0.45 25 340 1200 20 0.165 140 50 32 + + E2 98 2.7 0.45 26 3401280 20 0.165 140 50 32 + + E3 95 3 0.41 23 340 1110 20 0.165 130 4534 + + E4 85 3.3 0.4 23 340 1300 20 0.165 130 45 34 + + CE1 98 2.1 0.4510 65 10000 80 0.165 160 170 1.5 − − CE2 110 1 0.45 65 65 80 80 0.165130 170 2.8 − − CE3 100 2 0.45 25 37 1200 150 0.165 160 190 1.5 − − CE4115 0.97 >2 70 20 50 >5000 12 − − Key to winding performance andhandling and to processing performance of films: ++: no tendency tostick to rollers or to other mechanical parts, no blocking problems onwinding or during processing on packaging machinery, low productioncosts −: tendency to stick to rollers or to other mechanical parts,blocking problems on winding or during processing on packagingmachinery, high production costs due to complicated handling of film inthe machinery

What is claimed is:
 1. A sealable, flame-retardant, coextruded,biaxially oriented polyester film with one matt side and with at leastone base layer B based on a thermoplastic polyester, and with a sealableouter layer A, and also with a matt outer layer C, wherein a flameretardant is present in at least one layer, wherein the sealable outerlayer A has a minimum sealing temperature of about 110° C. and a sealseam strength of at least 1.3 N/15 mm, and the topographies of the twoouter layers A and C have the following features: Sealable outer layerA: R₃<about 30 nm Value measured for gas flow from about 500 to about4000 s Non-sealable, matt outer layer C: about 200 nm<R_(a)<about 1000nm Value measured for gas flow <about 50 s.
 2. The film as claimed inclaim 1, wherein the sealable outer layer A comprises an amorphouscopolyester which has been built up from ethylene terephthalate unitsand ethylene isophthalate units and from ethylene glycol units.
 3. Thefilm as claimed in claim 2, wherein the amorphous copolyester of thesealable outer layer A comprises from 40 to 95 mol % of ethyleneterephthalate and from 60 to 5 mol % of ethylene isophthalate.
 4. Thefilm as claimed in claim 2, wherein the amorphous copolyester of thesealable outer layer A contains from about 50 to about 90 mol % ofethylene terephthalate and from 50 to 10 mol % of ethylene isophthalate.5. The film as claimed in claim 2, wherein the amorphous copolyester ofthe sealable outer layer A contains from about 60 to about 85 mol % ofethylene terphthalate and from 40 to 15 mol % of ethylene isophthalate.6. The film as claimed in claim 1, wherein the matt outer layer Ccomprises a blend or a mixture made from two components I and II.
 7. Thefilm as claimed in claim 6, wherein the matt outer layer furthercomprises additives in the form of inert inorganic antiblocking agents.8. The film as claimed in claim 1, wherein the concentration of theflame retardant is in the range from about 0.5 to about 30.0% by weightbased on the weight of the respective layer of the polyester used. 9.The film as claimed in claim 8, wherein the concentration of the flameretardant is in the range from about 1.0 to about 20.0% by weight,. 10.The film as claimed in claim 1, wherein the flame retardant is selectedfrom organophosphorus compounds.
 11. The film as claimed in claim 1,wherein the flame retardant is selected from one or more ofcarboxyphosphinic acids, anhydrides of these, and dimethylmethylphosphonate.
 12. The film as claimed in claim 1, wherein recycledmaterial is present at a concentration of up to about 60% by weight,based on the total weight of the film.
 13. A process for producing afilm according to claim 1 with at least one base layer B based on athermoplastic polyester, and with a sealable outer layer A, and alsowith a matt outer layer C, wherein at least one layer comprises a flameretardant, wherein melts corresponding to the individual layers of thefilm are coextruded through a flat-film die, the resultant film is drawnoff on one or more rollers for solidification, and then the film isbiaxially stretched (oriented) and the biaxially stretch film isheat-set.
 14. The process as claimed in claim 13, wherein the flameretardant is added by way of masterbatch technology, and where themasterbatch has been precrystallized or predried or precrystallized andpredried.
 15. The process as claimed in claim 14, wherein the percentageratio by weight of flame retardant to thermoplastic in the masterbatchis from about 60:about 40 to about 10:about
 90. 16. The process asclaimed in claim 14, wherein the masterbatch also comprises a hydrolysisstabilizer in the form of a phenolic stabilizer, one or more of alkalimetal stearate, alkaline earth metal stearates, alkali metal carbonateand alkaline earth metal carbonate, in amounts of from about 0.05 toabout 0.6% by weight.
 17. The process as claimed in claim 16, whereinthe hydrolysis stabilizer is present in an amount of from about 0.15 toabout 0.3% by weight, and with a molar mass of more than about 500g/mol.
 18. The process as claimed in claim 13, wherein the film iscorona- or flame-treated on the surface layer intended for treatment.19. A method of making an interior decoration, a display, a placards, aprotective glazing, a shop outfit, a promotional requisite, a laminatingmedium, an exterior cladding, a protective covering, an illuminatedadvertising profile, or a blind, which comprises converting a film asclaimed in claim 1 into an interior decoration, a display, a placards, aprotective glazing, a shop outfit, a promotional requisite, a laminatingmedium, an exterior cladding, a protective covering, an illuminatedadvertising profile, or a blind.